Registration of image data with the location or orientation of an imaged object is one of the biggest challenges for an image-assisted navigation procedure, for instance during a surgical operation. The precision of the entire navigation procedure, and thus the surgical result, strongly depend on the registration quality.
There are numerous registration procedures that typically require auxiliary devices or manual interaction of the surgeon. Common examples are paired-points registration by matching single points obtained from pre-operative image data manually or automatically with corresponding points on the patient, and surface registration procedures by matching a defined surface of the pre-operative image data with the corresponding surface of the patient.
Many registration procedures are based on so-called fiducials. Fiducials are reference markings that are attached to the anatomy of the patient before imaging and that also show up in the image data. Conventionally, the fiducials themselves form the transitional items which allow registration between the virtual data space and the physical world. The fiducials have known relative positions to a navigational patient tracker and are attached together with the tracker to the anatomy to be tracked. By knowing the fiducial positions with respect to the tracker, the patient anatomy can automatically be registered for navigation. Prior to registration, it is typically required that at any part that needs to be tracked, and where a tracker is attached, a set of points needs to be identified for matching purposes.
Another registration procedure is based on attaching dedicated trackers to the imaging device and recording their position in space relative to the navigation system. This procedure requires a special adaptation of the imaging device.
A manually performed registration procedure based on fiducials attached to the patient prior to pre-operative imaging is disclosed in U.S. Pat. No. 6,226,548 B1. An identification superstructure including a reference arc and a fiducial array is connected to an attaching device which is fixed to a body (bony) part of a patient. The fiducial array has the function of identifying the location of the superstructure and the body part during imaging (e.g., by a Computer Tomography (CT) scan or Magnetic Resonance Imaging (MRI)) and is detectable by a navigation system.
After the pre-operative scan, the identification superstructure is removed from the attaching device and placed back on the patient in the operating room. The fiducial array and, therefore, the location of the body part is registered in a surgical navigation system by manually pointing to the fiducials with a pointer device of the navigation system. Such manually performed registration procedures are in general time-consuming and imprecise.
Automatic registration procedures are also common particularly in combination with intra-operative imaging. An intra-operative imaging scenario based on a tracking of the imaging device is described in U.S. Pat. No. 7,251,522 B. Before the surgical operation, a calibrating procedure has to be performed in order to define a spatial relationship between a 3D imaging volume and the imaging device, with the calibration device being centered in the 3D imaging volume. The calibration device is removed after the calibration procedure. Then, image data of the patient body are scanned and both the patient and the imaging device are continuously tracked by the navigation system in order to establish a spatial relationship between the patient and the image data and to perform automatic registration.
A similar pre-calibrating processing is described in US 2004/0127788 A1. A scanner calibrator is used to pre-calibrate a scanner by determining a transformation for mapping points and coordinates in images taken by the scanner to the position of a scanner tracker. After the scanner calibrator is removed, a transformation for mapping images from the scanner taken during an interventional procedure to a patient tracker is determined based on the locations of the patient tracker and the scanner tracker. Thereby, the images are registered to the patient. Thus, this registration procedure requires a tracker attached to the imaging device. Further, an additional calibration process is necessary before starting a surgical operation.
It is known that image registering systems as known from U.S. Pat. No. 7,251,522 B and US 2004/0127788 A1 use trackers mounted on imaging devices, such as a CT scanner or a MRI device, are limited in their possibilities. Only the pre-calibrated imaging device can be used for a surgical operation, since otherwise, it is not possible to map image data. Moreover, the imaging device needs to be recalibrated when the imaging device or the tracker thereof has been moved.
In U.S. Pat. No. 6,527,443 B1 another technique for a navigation-assisted treatment of a body part is disclosed. The technique comprises providing a C-arm with a reference structure including a fiducial array and a tracker to be tracked by a navigation system. A two-dimensional (2D) X-ray image of both the body part of the patient and the reference structure is taken, and the location of the body part is determined from the X-ray image and the tracking data generated by the navigation system. The patient needs to be fixed during the operation, since no patient tracking takes place.
A further automatic registration procedure is disclosed in U.S. Pat. No. 6,738,656 B1. The automatic registration is performed by attaching an array of imaging phantoms or fiducials forming a reference unit to the patient prior to imaging. Then a tracker is attached to the reference unit in a fixed spatial relationship with respect to the array of fiducials. The automatic registration process locates the fiducials on the prerecorded scan images, and, based on the known spatial relationship between the fiducials and the tracker, automatically generates a transformation function.
The automatic registration procedures discussed above allow for registration of the imaged anatomy with a coordinate system of the navigation system, but need complicated adaptations, trackers to be attached to the imaging device, and attachment of fiducials or of bulky phantoms that are difficult to install and hinder surgery.